Comma

From AudioLexic

In music theory, a comma is a small or very small interval between two enharmonic notes tuned in different ways. For example, an A flat tuned as a major third below C in just intonation, and a G sharp tuned as a major third above E, will not be exactly the same note. The difference between those notes, the diesis, is almost a quarter tone, easily audible.

All of the intervals mentioned below are presumed to be tuned in just intonation.

Commas are frequently used in the description of musical temperaments, where they describe minute tuning differences that are eliminated by that tuning system. For instance, twelve-tone equal temperamenttempers out the Pythagorean comma by narrowing each fifth slightly.